[B-A and B-Z transitions in deoxyoligoduplexes containing 4- and 5- methylcytosine]

Self-complementary oligodeoxynucleotides: GGACCCGGGTCC, GGA4mCCCGGGTCC, GGA5mCCCGGGTCC, CGCGCGCG, CG4mCGCGCG, CG5mCGCGCG were synthetized to study the contribution of methyl groups into the energetics of the three known cooperative transitions in DNA: helix-coil, B-A and B-Z With the use of circular dichroism and absorbtion methods the profiles of the above transitions were obtained by variation of temperature (helix-coil), trifluoroethanol fraction (B-A), NaCl and trifluorethanol contents (B-Z). On the basis of the transition widths and shifts of the transition points due to the methylations the energetics of the methyl groups was estimated. 5mC stabilizes the B form relatively the A form by 0.33 kcal/mol; while 4mC by 0.5 kcal/mol. In the B-Z transition 5 mC stabilizes the Z form by 0.28 kcal/mol relatively the B form; 4mC stabilizes also the Z form although by 0.14 kcal/mol only. Thus, these naturally occurring modifications could modulate substantially the ability of a DNA piece to shift into the A or Z form.